In Justice League #54, by Joshua Williamson, Xermanico, Romulo Fajardo Jr. and Tom Napolitano, the heroes stumble into a wasteland where Dr. Arkham used radiation to experiment on some of Batman's enemies. The results are even more twisted versions of some of the Darkest Knight's most brutal enemies. One of the supervillains seen is Killer Croc. Having been around since 1983, Killer Croc has gone through some dramatic changes in both appearance and demeanor.
Killer Croc had his first full appearance in Detective Comics #524 in 1983. Created by Gerry Conway, Don Newton and Gene Colan, Killer Croc was originally a sideshow wrestler stricken with a rare genetic disorder resulting in his crocodile-like appearance. The change drove him insane and he adopted the "Killer Croc" name as well as turning to a life of crime. Killer Croc was also on the Suicide Squad where his strength and fierceness helped him survive many missions.
Killer Croc has always been a formidable foe for Batman and has proven how ferocious he can be time and again. After Bane frees the inmates from Arkham Asylum During the events of Knightfall, Croc has a beef with Bane. When Croc finally confronts him he shows how ferocious he is and holds his own against Bane even after having his arm broken. The fight ends as a draw which is saying something considering Bane is the supervillain that broke Batman's back.
Up to this point, Croc is still a human with a genetic disorder who is mad and thrashes out at the world. During a two-issue storyline in Batman #521-522, by Doug Moench, Kelley Jones, John Beatty, and Gregory Wright, Batman discovers that Swamp Thing lures Killer Croc into the swamps to offer him a safe place where he can give in to his animalistic side and live free from other humans. Swamp Thing believes that if Croc were to abandon his human side he would be happier and find peace.
The idea of Killer Croc becoming more bestial is really explored in the 2002 Batman storyline "Hush," by Jeph Loeb, Jim Lee, and Scott Williams. In this story, a mysterious new villain vows to take down the Dark Knight at any cost. Many baddies from Batman's rogue's gallery appear including Killer Croc. In "Hush," Croc is infected with a virus that accelerates his rate of devolution making him more bestial and animalistic. This is the first time Croc is seen as being out of control and mindless and looking more like a lizard-beast than a man.
In the aftermath of "Hush," Killer Croc is being treated by Dr. Maria Belleza who is trying to cure him and reverse the mutation caused by the Hush virus. She ultimately fails and Croc, angry and still under the virus's influence, lets his base instincts overcome him and he devours Belleza. Giving in to his cannibalistic urges is quite a character change from Croc's original portrayal of a wrestler with a disease.
From being an angry wrestler to a cannibalistic killer to a reformed villain, Killer Croc has been through the full spectrum of character changes. His recent appearances in the DCU suggest Killer Croc is still on the path of being reformed, but with the threat of his base urges constantly bubbling to the surface it could only be a matter of time before Killer Croc is back to his supervillain ways.